This invention relates generally to spark plugs and other ignition devices used in internal combustion engines and, more particularly, to such ignition devices having noble metal firing tips. As used herein, the term xe2x80x9cignition devicexe2x80x9d shall be understood to include spark plugs, igniters, and other such devices that are used to initiate the combustion of a gas or fuel.
Within the field of spark plugs, there exists a continuing need to improve the erosion resistance and reduce the sparking voltage at the spark plug""s center and ground electrodes. To this end, various designs have been proposed using noble metal electrodes or, more commonly, noble metal firing tips applied to standard metal electrodes. Typically, the firing tip is formed as a pad or rivet which is then welded onto the end of the electrode.
Platinum and iridium alloys are two of the noble metals most commonly used for these firing tips. See, for example, U.S. Pat. No. 4,540,910 to Kondo et al. which discloses a center electrode firing tip made from 70 to 90 wt % platinum and 30 to 10 wt % iridium. As mentioned in that patent, platinum-tungsten alloys have also been used for these firing tips. Such a platinum-tungsten alloy is also disclosed in U.S. Pat. No. 6,045,424 to Chang et al., which further teaches the construction of firing tips using platinum-rhodium alloys and platinum-iridium-tungsten alloys.
Apart from these basic noble metal alloys, oxide dispersion strengthened alloys have also been proposed which utilize combinations of the above-noted metals with varying amounts of different rare earth metal oxides. See, for example, U.S. Pat. No. 4,081,710 to Heywood et al. In this regard, several specific platinum and iridium-based alloys have been suggested which utilize yttrium oxide (Y2O3). In particular, U.S. Pat. No. 5,456,624 to Moore et al. discloses a firing tip made from a platinum alloy containing  less than 2% yttrium oxide. U.S. Pat. No. 5,990,602 to Katoh et al. discloses a platinum-iridium alloy containing between 0.01 and 2% yttrium oxide. U.S. Pat. No. 5,461,275 to Oshima discloses an iridium alloy that includes between 5 and 15% yttrium oxide. While the yttrium oxide has historically been included in small amounts (e.g.,  less than 2%) to improve the strength and/or stability of the resultant alloy, the Oshima patent teaches that, by using yttrium oxide with iridium at  greater than 5% by volume, the sparking voltage can be reduced.
The present invention is based on the finding that reduced erosion and lowered sparking voltages can be achieved at much lower percentages of yttrium oxide than are disclosed in the Oshima patent by incorporating the yttrium oxide into an alloy of tungsten and platinum.
Thus, in accordance with the invention there is provided an ignition device having both a ground and center electrode, wherein at least one of the electrodes includes a firing tip formed from an alloy containing platinum, tungsten, and yttrium oxide. Preferably, the alloy is formed from a combination of 91.7%-97.99% platinum, 2%-8% tungsten, and 0.01%-0.3% yttrium, by weight, and in an even more preferred construction, 95.68%-96.12% platinum, 3.8%-4.2% tungsten, and 0.08%-0.12% yttrium. The firing tip can take the form of a pad, rivet, ball, wire, or other shape and can be welded in place on the electrode.
In addition to good erosion resistance and a reduced sparking voltage, a firing tip constructed from the platinum/tungsten/yttrium oxide alloy provides a number of additional advantages, including good welding and forming characteristics, as well as a minimal occurrence of oxidation of the alloy at the weld area.